Porous metal structures, ceramic monoliths and the so-called cermets, that is, composites of ceramic and metal materials, have been known for many years. These materials have been useful because of their inherent catalytic properties as well as their ability to serve as catalyst support structures; for example, porous nickel sheets have been used in battery electrodes, particularly as components in fuel cell electrodes, while ceramic and cermet materials have been used as supports for metals and metal oxides which are active as catalysts in chemical reactions. In recent years there has been increasing interest in the use of ceramic and cermet materials as supports for catalytic materials used in treating automobile exhaust emissions. See for example U.S. Pat. No. 3,444,925 to Johnson which discloses ceramic and cermet honeycomb structures and their use in automobile exhaust gas catalytic converters; U.S. Pat. No. 3,492,098 to De Palma et al. in which a multi-layer structure, which includes a ceramic layer and an alumina layer, is used as a support for a third layer of catalyst; and U.S. Pat. Nos. 3,471,413 and 3,492,148 both to Hervet which also disclose multi-layer structures including a porcelain layer, an alumina layer and a catalyst layer.
The ideal catalyst support should combine physical strength, high porosity to permit rapid fluid transport to and out of the support, a high surface area on which catalytic materials can be deposited and ability to withstand elevated operating temperatures. The porous catalyst supports heretofore known have often been satisfactory as to some of these properties, but none of the catalyst supports heretofore known have been totally satisfactory for uses under stringent operating conditions, for example, in auto exhaust catalytic converters.
The present invention provides an improved porous composite structure which exhibits all of the aforementioned desirable properties of strength, internal porosity, high surface area and ability to withstand elevated temperature service.